1. Field of the Invention
The present invention relates to a method and an apparatus for identifying individuals based upon iris textures extracted from images of individuals' eyes.
2. Description of the Prior Art
Individual identification technology is known which extracts iris textures from images of eyes including pupils and irises and identifies individuals based upon the extracted irises textures. For example, U.S. Pat. No. 5,291,560 has disclosed a typical individual identification system. Such an individual identification system converts the irises textures laid in eye images into irises codes, thus carrying out individual identification by comparing such irises codes. Accordingly, the individual identification system must acquire the positions of the irises or the outlines thereof.
In general, shapes of pupils and irises of human beings are circular. Therefore, the individual identification system prepares an imaginary circle circumscribed or inscribed with respect to the pupil of an individual. Likewise, the system prepares another circle with respect to the iris of the individual. Such a preparation of circles serves to establish a polar coordinate system on the eye image of the individual, wherein information on the position of the iris is given based upon the polar coordinate system.
Herein, the circle representative of an iris of a human being is defined by a center (Xc, Yc) and a radius R that maximize a value which is given by integrating values of pixels (X, Y) in the eye image along the circumference of the circle and differentiating the result of the integration by the radius R. Establishment of a polar coordinate system using the center (Xc, Yc) and the radius R, enables the individual identification to be immune to the influence of noise. In the polar coordinate system, the iris texture is divided into a plurality of areas, wherein the areas are converted into iris codes. The individual identification is carried out by comparing the iris code of an individual with the iris codes of other individuals. Therefore, in the individual identification system, preparing a circle for a human being's iris is essential to obtain an iris code representative of the iris.
Incidentally, pupils and irises of animals, e.g., horses are elliptical, in general. Accordingly, the conventional individual identification system cannot acquire an iris texture of a horse using a circle. For example, as shown in FIG. 12(a), a pupil 100 of a human being is approximately circular, and remains circular regardless of change thereof in opening. Similarly, the iris 200 is roughly circular and its shape does not depends upon opening. These make it possible to provide optimum circles representative of the iris 200, or the boundary between the pupil 100 and the iris 200 and the boundary between the iris 200 and the sclera 300. In contrast, as shown in FIG. 12(b), a pupil 400 of a horse is elliptical, and also the ratio of the line of apsides and the minor axis thereof depends upon change in opening. Consequently, no optimum circles representative of the pupil 400 or the iris 500 can be given. In other word, neither the boundary between the pupil 400 and the iris 500 nor the boundary between the iris 500 and the sclera 600 are obtained. This makes it impossible to carry out individual identification of a horse.
In addition, there exists an irial granule 700 along the boundary between the pupil 400 and the iris 500, in particular along the upper part thereof, which is common to the ruminants such as cattle, goats, and sheep. Further, the shape of the irial granule 700 depends upon individuals. Accordingly, accurate detection of the pupil 400 or the iris 500 is required to be immune to the presence of the irial granule 700.
Even shapes of human beings' eyes in eye images change according to photographic conditions, and more exactly, the shapes of human beings' pupils and irises sometimes become elliptical. Therefore, assuming that the pupil and the iris change into an elliptical shape, use of circles to approximate the outlines of the pupil 100 and the iris 200 deteriorates the accuracy with which individuals can be identified.
As shown in FIGS. 13(a)-(c), there might be several circles 900a.about.900c similar to one elliptical pupil 800. This is because the state of the outline of the pupil 800, e.g., brightness and contrast thereof, allows such different circles 900a.about.900c to arise. In FIGS. 13(a)-(c), the pupil 800 is in contact with the circles 900a.about.900c by the sides, top, and bottom thereof, respectively. In addition to these circles 900a.about.900c, there appear to exist other circles. Similarly, an iris might have a plurality of circles.
As discussed above, the conventional individual identification system acquires a position, or an outline representative of an elliptical pupil and an elliptical iris by preparing respective circles which are approximate thereto. Accordingly, it is difficult to acquire an accurate outlines or an accurate positions of an elliptical pupil and an elliptical iris, which deteriorates the performance of individual identification systems.